A FilterAtomicReader for updating facets ordinal references,
based on an ordinal map. You should use this code in conjunction with merging
taxonomies - after you merge taxonomies, you receive an DirectoryTaxonomyWriter.OrdinalMap
which maps the 'old' payloads to the 'new' ones. You can use that map to
re-map the payloads which contain the facets information (ordinals) either
before or while merging the indexes.